The data models used at the logic layer are classified into two types: one is mainly used for database design, which can be understood by general users and is similar to the way people think. Such models include (ERM). The other is to model data from the perspective of computer systems, making data more suitable for Computer Representation. The model is mainly used for DBMS implementation, for example. A designer's approach to building a database model is usually to first use a E-R model to model data at a high level, and then convert it into a relational model. The data model used at the physical layer is called the physical data model.
It consists of three parts.
(DDL), used to define the database mode; 2) (DML), used to query and update databases; 3) (DCL), used to manage data permissions.
It consists of three parts. In a relational model, the real-world entities and the relationships between entities are expressed.
The logic-based definition is called. Due to the use of different variables, relational calculus is divided into tuples relational calculus and domain relational calculus.
. Entity integrity and referential integrity are integrity constraints that must be met by the relational model ..
. Therefore, the data definition function of SQL includes schema definition, table definition, view definition, and index definition. SQL generally does not support modifying schema definitions, modifying view definitions, and modifying index definitions. If you want to modify these objects, you can only delete them and then recreate them.